The invention relates to a method for the finishing of a crankshaft bearing bore in a cylinder crankcase of an internal combustion engine. The invention also relates to a machining installation suitable for carrying out the method.
The cylinder crankcase, which is often also referred to simply as the “crankcase” or “engine block”, is an integral part of combustion engines or internal combustion engines, as are used for example in passenger cars or trucks, aircraft, ships or stationary installations. The most widely encountered design is that of multi-cylinder engines, the pistons of which are connected by means of connecting rods to the rotating crankshaft, which passes on the power generated by the engine to wheels, propellers, generators or the like.
The crankshaft lies below the cylinders in in-line or V-type engines and between the cylinders in opposed-cylinder engines, and is supported on the cylinder crankcase at the bearing points of the crankshaft bearing bore. The bearing points generally take the form of plain bearings, sometimes also antifriction bearings. In order to ensure a high degree of concentricity of the crankshaft, and consequently to minimize undesired effects of wear and vibrations during operation, the crankshaft must meet high dimensional tolerance requirements. The bearing points also have close tolerances with regard to their size and position in the cylinder crankcase.
The relatively close tolerances of the important parts of an internal combustion engine and of a cylinder crankcase are a result of the complex functions of these parts. The shape, size and surface topography of the cylinder bore thereby crucially determine the amount of wear, the friction and the oil consumption and emission values of the combustion engine and, as a result of the friction, also the output and efficiency. The compressed combustion-chamber volume particularly determines the compression of the engine. This has effects on the output and, by the combustion sequence, also an influence on the emission values and on engine noises.
Among other reasons, the exact position of the crankshaft in relation to the pistons or the cylinder bores is important for reducing wear on the engine parts that are subjected to high loading. These are, in particular, pistons/piston pins, connecting rods and bearings of the connecting rod on the crankshaft and also the mounting of the crankshaft in the cylinder crankcase. As far as the position of the bore axis is concerned, not only the absolute spatial position but also the angular position or orientation thereof play a part. For example, in the case of manual transmissions with a clutch, the crankshaft and the input shaft of the transmission run with each other. Therefore, a coaxiality of the two shafts is decisive here for a long service life.
As far as the crankshaft mounting is concerned, the coaxiality of the bearing points in relation to one another is also important. The bearing points should lie in line as well as possible, in order that the crankshaft can run concentrically and rests substantially uniformly at all the bearing points.
The workpiece region around a crankshaft bearing bore is generally made up of two individual parts. These are most frequently the bearing webs of the cylinder crankcase on the one hand and the bearing covers screwed onto them on the other hand. Alternatively, the crankshaft bearing bore may also lie at the interface of two cylinder crankcase halves, such as for example in the case of the opposed-cylinder engine. The individual parts are generally pre-machined in the unfitted state, in that half-round surface portions are respectively produced on the bearing webs of the cylinder crankcase and on the bearing covers. In a subsequent process step, the individual parts are screwed together, so that a substantially cylindrical bore portion is respectively obtained from the half-round surface portions in the region of a bearing point. A number of bore portions lying spaced apart one behind the other in series then provide the overall crankshaft bearing bore.
The bore prepared in this way is then subjected to a multi-stage finishing. In order to be able to achieve the tolerances in the production of cylinder crankcases, nowadays a combination of one or more drilling operations and/or frictional operations and one or more subsequent honing operations is generally used. The multiplicity of operations are provided because irregularities, for example caused by the casting of the cylinder crankcase, make it necessary for relatively great amounts of material, of several tenths of a millimeter or more, to be removed, but at the same time dimensional and positional tolerances may lie in the lower micrometer range, and consequently require high-precision machining.
The machining stages with the aid of tools with geometrically defined cutting edges (drilling tools or frictional tools) have in this case two main tasks. One task is to establish the position of the overall crankshaft bearing bore with reference to a system of coordinates that is fixed in relation to the workpiece, for example with reference to a corresponding reference surface on the cylinder crankcase. Therefore, by means of finishing with geometrically defined cutting edges, the desired position of the bore axis that is prescribed for the workpiece is produced. The other task of the finishing by means of tools with geometrically defined cutting edges is that of positioning the individual bearing points in relation to one another, which among those skilled in the art is also referred to as the “coaxiality” of the crankshaft bearing bore. The total amounts of material removal with respect to the bore diameter in the case of these operations typically lie in the range of several tenths of a millimeter, sometimes also one millimeter or more.
The finishing by means of tools with geometrically defined cutting edges is followed by a honing of the crankshaft bearing bore, that is to say finishing with a tool or a number of tools with geometrically undefined cutting edges. The honing particularly achieves the required final quality with regard to diameter tolerance, cylindrical shape and surface roughness, so that the tolerances with regard to the prescribable desired size and the prescribable desired structure of the bore inner surface can be maintained.
It is intended that the position and coaxiality of the crankshaft bearing bore should not be changed during the honing, since they have already been established by the preceding operations. Therefore, an articulated or compliant coupling between the honing tool and the honing spindle is provided for the honing, in order that the honing tool can follow the bore that has already been prescribed with regard to its position, without actively changing the position thereof. The amounts of material removal with respect to the bore diameter in the honing operation typically lie in the range of well below 100 μm.
EP 0 968 069 B2 describes a drilling machine which can be used in the mass production of cylinder crankcases for drilling the crankshaft bearing bores.
DE 196 34 415 B4 discloses a honing tool which can be used, for example, when honing crankshaft bearing bores or other bores that are divided into portions and have bore portions arranged in line in series. The honing tool has at least one honing region, which can be set according to the desired size before the beginning of machining and has a cutting region and calibrating region each with at least one cutting coating, and a guiding cutting region with at least one cutting coating, the guiding cutting region being radially expandable.